UTM professor Robert Reisz and his team unearthed this skull of adult and complete embryo of the Early Jurassic (190-million-year-old) dinosaur Massospondylus in the South African nesting site. (Credit: Photo courtesy of Robert Reisz)

ScienceDaily (Jan. 23, 2012) — An excavation at a site in South Africa has unearthed the 190-million-year-old dinosaur nesting site of the prosauropod dinosaur Massospondylus-revealing significant clues about the evolution of complex reproductive behaviour in early dinosaurs. The newly unearthed dinosaur nesting ground predates previously known nesting sites by 100 million years, according to study authors.
A new study led by University of Toronto Mississauga paleontologist Robert Reisz, with co-author, Professor David Evans of ecology and evolutionary biology and the Royal Ontario Museum, along with a group of international researchers, describes clutches of eggs, many with embryos, as well as tiny dinosaur footprints, providing the oldest known evidence that the hatchlings remained at the nesting site long enough to at least double in size.
At least 10 nests have been discovered at several levels at this site, each with up to 34 round eggs in tightly clustered clutches. The distribution of the nests in the sediments indicate that these early dinosaurs returned repeatedly to this site, a behaviour known as nesting fidelity, and likely assembled in groups to lay their eggs, (colonial nesting), the oldest known evidence of such behaviour in the fossil record. The large size of the mother, at six metres in length, the small size of the eggs, about six to seven centimetres in diameter, and the highly organized nature of the nest suggest that the mother may have arranged them carefully after she laid them.
"The eggs, embryos, and nests come from the rocks of a nearly vertical road cut only 25 metres long," said Reisz, a professor of biology at U of T Mississauga. "Even so, we found ten nests, suggesting that there are a lot more in the cliff, still covered by tons of rock. We predict that many more nests will be eroded out in time as natural weathering processes continue."
The fossils were found in sedimentary rocks from the Early Jurassic Period in the Golden Gate Highlands National Park in South Africa. This site has previously yielded the oldest known embryos belonging to Massospondylus, a relative of the giant, long-necked sauropods of the Jurassic and Cretaceous periods.
"Even though the fossil record of dinosaurs is extensive, we actually have very little fossil information about their reproductive biology, particularly for early dinosaurs," said Evans (pictured left, bottom, with Reisz, above), associate curator of vertebrate palaeontology at the Royal Ontario Museum. "This amazing series of 190 million year old nests gives us the first detailed look at dinosaur reproduction early in their evolutionary history, and documents the antiquity of nesting strategies that are only known much later in the dinosaur record."
The study, co-authored by Drs. Hans-Dieter Sues (Smithsonian Institute, U.S.), Eric Roberts (James Cook University, Australia), and Adam Yates (University of the Witwatersrand, South Africa), is published in the Proceedings of the National Academy of Sciences.
An exhibition currently on display at the Royal Ontario Museum until May 2012, Dinosaurs Eggs and Babies: Remarkable Fossils from South Africa, features the oldest fossilized dinosaur eggs with embryos ever found, as well as other impressive discoveries

Reisz, R. R., D. C. Evans, et al. (2012). "Oldest known dinosaurian nesting site and reproductive biology of the Early Jurassic sauropodomorph Massospondylus." Proceedings of the National Academy of Sciences.

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The extensive Early Jurassic continental strata of southern Africa have yielded an exceptional record of dinosaurs that includes scores of partial to complete skeletons of the sauropodomorph Massospondylus, ranging from embryos to large adults. In 1976 an incomplete egg clutch including in ovo embryos of this dinosaur, the oldest known example in the fossil record, was collected from a road-cut talus, but its exact provenance was uncertain. An excavation program at the site started in 2006 has yielded multiple in situ egg clutches, documenting the oldest known dinosaurian nesting site, predating other similar sites by more than 100 million years. The presence of numerous clutches of eggs, some of which contain embryonic remains, in at least four distinct horizons within a small area, provides the earliest known evidence of complex reproductive behavior including site fidelity and colonial nesting in a terrestrial vertebrate. Thus, fossil and sedimentological evidence from this nesting site provides empirical data on reproductive strategies in early dinosaurs. A temporally calibrated optimization of dinosaurian reproductive biology not only demonstrates the primary significance of the Massospondylus nesting site, but also provides additional insights into the initial stages of the evolutionary history of dinosaurs, including evidence that deposition of eggs in a tightly organized single layer in a nest evolved independently from brooding.